#include<stdio.h>
int n,k,j,nr[26][26];
double s=1,p[26],st[26][26];

int main()
{
freopen("dezastru.in","r",stdin);
freopen("dezastru.out","w",stdout);

scanf("%d%d",&n,&k);

int i;

for(i=1;i<=n;++i)
   {
   scanf("%lf",&p[i]);
   st[i][0]=1;
   nr[i][0]=1;
   s=s*p[i];   
   } 
if(n==k)
  printf("%.6lf\n",s);
else
{   
st[0][0]=1;
nr[0][0]=1;
for(i=1;i<=n;++i)
   for(j=1;j<=i;++j)
      {
      st[i][j]=st[i-1][j]+st[i-1][j-1]*p[i];
      nr[i][j]=nr[i-1][j]+nr[i-1][j-1];
      }
s=st[n][k]/nr[n][k];
printf("%.6lf\n",s);
}
return 0;
}